According to a certain estimate, the depth M(t), in centimeters, of the water in a different tank at t hours past 3:00 in the morning is given by $M(t) = -15(t - 4)^2 + 450$ for $0 ≤ t ≤ 8$. According to this estimate, at what time in the morning does the depth of the water in the tank reach its maximum?